When pulling a cork out of a bottle with a traditional corkscrew, for example a ‘sommelier knife’, the problem often arises that the cork crumbles or breaks off completely in the middle. In particular, but not exclusively, corks of a lower quality and older corks exhibit this problem.
As a result of this, part of the cork can be left behind in the bottle neck, such that the content of the bottle cannot be poured out until this part has been removed with difficulty. Cork can also get into the wine.
Also with traditional corkscrews, a substantial force is often required to pull a cork out of a bottle.